In a method for determining whether multiple power outlets in a 3-wire power line connection are connected to a same outer wire (P1 or P2), the hot terminal of a first power outlet is identified and a first zero cross time of the hot terminal to which AC voltage is applied is detected. The hot terminal of a second power outlet is identified, and second zero cross time of the hot terminal to which AC voltage is applied is detected. It is determined that the first power outlet and the second power outlet are connected to the same power line if the time elapsed AT from the first zero cross time of the first power outlet to the second zero cross time of the second power outlet is an integral multiple of the AC voltage cycle.